The present invention relates generally to control systems and more specifically to feedback control systems using estimated back electromotive force(back EMF) as a feedback in the control loop.
Control systems are well known in the art and have been used for many years to control applications ranging from large chemical plants and manufacturing plants to aircraft, automobiles, and home appliances. Early control systems were implemented using analog circuits, however, with the advent of computers and microprocessors, many control systems are now implemented using digital circuits.
One type of control system is a feedback or closed loop control system. A feedback control system controls an output quantity by feeding back a measured value of the controlled quantity and using it to manipulate an input quantity(or command) so as to bring the value of the controlled quantity closer to a desired value.
Control systems are often used to control actuators which in turn perform a desired mechanical operation. Two types of actuators are parallel actuators and linear(or series) actuators. Parallel actuators generally drive a load along a pair of connection points while linear actuators generally drive a load along a single path or connection point. FIGS. 1A and 1B illustrate the difference between parallel and linear actuators.
FIG. 1A shows a load 1 driven by a parallel actuator 2. The dotted lines represent connection means such as chains, cables, belts, or the like connecting the parallel actuator 2 and load 1. Rotation of parallel actuator 2 induces rotation in load 1 as indicated by arrow 3. Those skilled in the art understand that there exist numerous types of parallel actuators.
FIG. 1B shows a linear actuator 4 anchored to a surface 5 and having an extendible arm 4A rotatably attached to a lever 6. Arm 4A extends and retracts causing lever 6 to rotate about pivot point 7. Those skilled in the art understand that there exist numerous types of linear actuators.
This brief description of parallel and linear actuators is for descriptive purposes only and is not intended to limit the scope of the invention.
Parallel and linear actuators are significantly different for control purposes since linear actuators often lack a rate feedback signal suitable for use by a digital feedback control system. Feedback of both rate and position information is essential for precision control of a high gain actuator. Parallel actuators easily provide rate and position feed back based on tachometer signals or the like. Most linear actuators do not provide a rate signal and therefore rate information must be extracted from the position signal which is often generated by a linear variable differential transformer(LVDT). This is costly and unsatisfactory for digital implementations.
FIG. 2 illustrates an LVDT position sensor. A known AC voltage is applied to primary winding 21A which induces AC voltage in secondary winding 21B. Core 22 moves relative to windings 21 indicative of the position of the actuator. Since the position of core 22 affects the electromagnetic coupling between the primary winding 21A and the secondary winding 21B in a known manner, the position of core 22, and thus the position of the actuator can be calculated by measuring the AC voltage in secondary winding 21B.
The prior art uses LVDT position sensors to provide both position and rate feedback data to an analog feedback control circuit. The analog circuit extracts rate information from the LVDT position signal. Although analog feedback circuits function properly they are undesirable since they require many components, cannot be easily adapted to different applications, and are difficult to monitor and test.
Attempts to create a digital equivalent of these analog circuits have been frustrated by difficulties in extracting a digital rate signal from the LVDT position signal. Digital equivalents of the analog design result in a noisy rate signal which is virtually useless. Improving this signal is costly since acceptable performance requires higher processing speed, higher resolution analog to digital(A/D) conversion, more filtering hardware, and more excitation stability for data sampling.
A digital feedback control loop for linear actuators would provide the many advantages discussed above. Clearly there exists a need for a digital feedback control system and method for controlling linear actuators which can be implemented at a reasonable cost.